1. Field of the Invention
An aspect of this disclosure is related to an oscillator circuit.
2. Description of the Related Art
Japanese Laid-Open Patent Publication 2008-147815, for example, discloses an oscillator circuit that generates an oscillator output signal using an oscillator as a resonator. The disclosed oscillator circuit includes two inverter circuits used as amplifier circuits and connected in parallel with the oscillator, and a timer for counting time from the start of oscillation. In the oscillator circuit, both of the inverter circuits are caused to operate for a predetermined period of time from the start of oscillation. After the predetermined period of time passes, only one of the inverter circuits is caused to operate and the other one of the inverter circuits is stopped. This configuration is intended to reduce the time necessary for the oscillation to stabilize and also to reduce power consumption.
Thus, according to the disclosed technology, boosting of the oscillator output signal is stopped when the time from the start of oscillation counted by the timer exceeds a predetermined value. However, characteristics of oscillators may vary depending on their types and individual differences. Therefore, if the timer is not properly set, boosting of the oscillator output signal may be stopped before the oscillator output signal stabilizes.
This problem may be prevented, for example, by using a method where boosting of an oscillator output signal is stopped when the amplitude of the oscillator output signal exceeds a predetermined value. Here, this method needs an amplitude detection circuit for outputting a signal Vs that changes when the amplitude of an oscillator output signal becomes greater than a predetermined value.
FIG. 1 illustrates an exemplary amplitude detection circuit that outputs a signal Vs. The amplitude detection circuit of FIG. 1 outputs the signal Vs using a pulse-width detection circuit. A comparator 4 of the amplitude detection circuit compares amplitude Vin of an oscillator output signal with a threshold voltage Vbias. Here, to monitor a high-frequency oscillation amplitude with a comparator as in this configuration, the comparator needs to have a high slew rate to be able to follow changes in input oscillation amplitude.
When Vout indicates the output voltage of a comparator, C indicates the capacitance of the comparator, and I indicates an electric current for charging and discharging the capacitance, the slew rate is expressed by formula (1) below.SR=dVout/dt−I/C  (1)
As is apparent from formula (1), to increase the slew rate, it is necessary to increase the electric current I. Accordingly, increasing the slew rate of a comparator increases the power consumption of the comparator.